[sdiy] Negative content on half wave rectifier

[Brock Russell]

My favorite absolute value circuit is from Ron Mancini, 
http://www.edn.com/contents/images/51503di.pdf, it's the last design 
idea. Not useful as a half wave rectifier though.

Brock

At 07:43 AM 22/07/2011, you wrote:
>as someone else pointed out, it seems you might have a circuit error 
>somewhere.
>
>I use circuit "6A" from that appnote you posted, its the best 
>absolute value circuit I've seen
>in terms of audio range performance. You can tap a half wave out of 
>that as well. If you study my
>"Muffy" guitar synthesizer over at wiseguysynth.com (Larry Hendry - 
>gone but not forgotten) you can
>see I used it there.
>
>If you need to absolutely get rid of a voltage below some point, you 
>can use active clamping instead
>of rectification.  This uses the same opamp and diode, but you run 
>your signal through a resistor and
>use the opamp/rectifier to shunt the circuit to ground or whatever 
>voltage you want.
>
>I'll try and find you an example circuit (or draw one...)
>
>h^) HARRY
>
>
>----- Original Message -----
>From: Justin Owen <juzowen at gmail.com>
>To: SDIY List <synth-diy at dropmix.xs4all.nl>
>Sent: Fri, 22 Jul 2011 09:43:26 -0400 (EDT)
>Subject: [sdiy] Negative content on half wave rectifier
>
>Hello,
>
>I was recently warned off using a single 1N4001 as a half wave 
>rectifier so I started playing with op-amp based versions. I settled 
>on the 'Improved Precision Half Wave Rectifier' in Fig. 3 here: 
>http://sound.westhost.com/appnotes/an001.htm
>
>This also shows up on P191 of Jung's op-amp cookbook.
>
>Jung quotes that, on an AC signal, you'll see a negative swing on 
>the output of approx. -0.6V (which I guess is an inverted diode 
>forward voltage drop?) - in reality I'm seeing between 0.8V and 1V 
>depending on the diodes I use (1N914s were best).
>
>Is there a way to completely get rid of the -V content or at least 
>improve these figures - so you're swinging between 0 and V?
>
>I have tried biasing the +v terminal but, for input summing of AC 
>and DC signals it's not a solution. To compensate for the +1V bias I 
>made the 0-5V CV swing between -1 and 4V but obviously that -1V 
>portion of the CV is now getting rectified and lower voltages don't 
>smooth up and down like they do without any bias (but with the -V 
>content) - they 'bump' on and off, I guess as the diodes turn on/off 
>- so it's causing more problems than it's solving.
>
>For the record, I can live with the negative content - but it is bugging me.
>
>Thanks,
>
>Justin
>
>
>
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